The best solution depends on the size and scale of the operation you are planning. Simple put, what’s best for a 500 person call center probably isn’t what’s best for a 50 person call center. Also one has to factor in the needs of the system, such as do you want agents to have direct numbers, or use extensions for customers to call back on. All these things come into play and effect costs, so to be honest you have not given enough detail to give a proper recommendation at this time.

At this time the employees in the Philippines will only be placing calls. There will only be four employees in total. Stateside we have around 300 employees and are already transitioning over to VoIP for Long Distance. The four employees do not need direct numbers or extensions to be called back on. Basically if they are successful in negoitiating a sale or achieve some level of interest the prospects information will be given over to our stateside account managers. Is this enough info or is more needed?

That’s enough info, but since the scenario in this case is very small scale, and is some what of what I would call a beta test, so you really don’t have much to play with. Initial costs need to be low, and without any long term commitment, so you can’t do things like get a VOIP PBX and hammer out a wholesale deal with one of the wholesale carriers I know to get the best long term deal since you don’t know if this will last long enough at this point to invest that kind of time and money at this point. So the best choice at this time would probably be with Vonage since they are the biggest SHO VIOP carrier, and thus probably the most stable. I'd get the equipment here with numbers local to the home office, arrange for internet access over there and ship the equipment them to them as it will work anywhere in the world that has a decent internet connection.

IAX is a protocol used by the Asterisk PBX system. It has benefits over SIP in that you can "trunk" calls, which you'd probably want to do in your environment.
Take a look at http://www.asterisk.org for information about Asterisk.

"With Vonage, if I purchase all the equipment over here and get local numbers, the per minute cost of the users in the Philippines would be equivalent to that of a domestic VoIP call, is this correct?"

That is correct, as that is one of their selling points, you can take the adaptor anywhere in the world and your local number and service will follow, and even if they didn’t want to do that, it would be very hard to get their system to track what country you where using it in anyway.

There may be databases for what IP addresses belong to what country, but they are far from accurate as I frequently find addresses that are supposed to be used in one country being used in some other country. So trying to bill in such a manor would be very problematic and error prone to say the least. So I doubt any one will anytime soon start billing VOIP calls based on the phones location since there is no way to be sure where it really is located.